In recent years, plastic films or sheets, or molded products thereof have generally been used as packaging materials for the purpose of preserving contents, from the viewpoints of, for example, transparency, light weight, and cost. Plastic films used for packaging foods, drugs, cosmetics, etc., are required to exhibit, for example, gas-barrier property to various gases, transparency, resistance to retort treatment, flexion resistance, flexibility, and heat sealability. Particularly, such a film is required to exhibit high gas-barrier property to oxygen and water vapor, from the viewpoint of maintaining the properties and characteristics of contents which are packaged by the film.
In general, thermoplastic plastic films exhibit only a moderate level of gas-barrier property. Therefore, hitherto, coating of such a film with a polyvinylidene chloride (PVDC) resin has generally been carried out as a technique for imparting gas-barrier property to the film. However, a PVDC-coated film produced through this technique poses a problem in that when the film, which contains halogen atoms, is incinerated, toxic substances (e.g., dioxins) are generated, which may cause environmental destruction.
As alternatives to such a PVDC-coated film, there have been disclosed, for example, the following films: a film formed of a saponification product of ethylene-vinyl acetate copolymer (EVOH resin), a polyvinyl alcohol (PVA)-coated film, an inorganic vapor-deposited film formed by depositing silica, alumina, or the like on a flexible polymer film, and a metal-foil-laminated film. However, such an EVOH resin film or PVA-coated film poses a problem in that water permeates the film under high-humidity conditions, and when the film is subjected to boiling or retort treatment, the oxygen-barrier property thereof is considerably deteriorated. Also, such an inorganic vapor-deposited film or metal-foil-laminated film, in which a gas-barrier layer is formed through deposition of a hard inorganic compound or formed of a metal, poses a problem in that cracks or pinholes are generated in the gas-barrier layer through flexion, resulting in considerable deterioration of gas-barrier property. In addition, such a deposited film requires a large-scale production apparatus (e.g., a vacuum deposition apparatus), as compared with the case of a coating film or a laminate film, and poses a problem in terms of production cost.
There have been disclosed a method for improving impact resistance or resistance to pinhole formation of an inorganic vapor-deposited film or a metal-foil-laminated film. Specifically, there has been proposed an aluminum-foil-containing pouch formed of a layered film including a thermoplastic resin film and an aluminum foil, wherein nylon films are stacked on both sides of the aluminum foil (see Patent Document 1). Also, there has been proposed a layered film exhibiting good impact resistance and resistance to pinhole formation, the film including a first layer (stretched nylon film), a second layer (metal foil), a third layer (coextrusion film including two LLDPE or LDPE layers and a nylon layer sandwiched therebetween), and a fourth layer (LLDPE film or LDPE film) (see Patent Document 2). Also, there has been proposed a layered film for packaging electronic parts, which film exhibits good moisture-proof property, resistance to pinhole formation, poking resistance, and antistatic property, and which includes an antistatic layer, a biaxially oriented polyethylene terephthalate layer, an aluminum foil layer, a biaxially oriented nylon layer, and an antistatic polyolefin layer, the layers being stacked in this order, wherein a gas-barrier layer formed of an aluminum-deposited layer, an aluminum-oxide-deposited layer, a silicon-oxide-deposited layer, and a vinylidene chloride resin layer is provided between the biaxially oriented polyethylene terephthalate layer and the biaxially oriented nylon layer (see Patent Document 3). Any of these means makes an attempt to improve impact resistance or resistance to pinhole formation by devising a layer structure, but requires a complicated layer structure (i.e., requires an intricate production process).
In connection with a coating technique for improving resistance to pinhole formation of an inorganic vapor-deposited film or a metal-foil-laminated film, there have been proposed a polyurethane resin exhibiting gas-barrier property and a gas-barrier film containing the resin (see Patent Document 4), and a coated film exhibiting gas-barrier property (see Patent Document 5). However, these do not necessarily satisfy both interfilm adhesion and gas-barrier property.
Meanwhile, there has been proposed a method for improving the gas-barrier property (to oxygen, carbon dioxide, etc.) of a composition containing an epoxy resin by increasing the amine nitrogen content of the composition (see Patent Documents 6 and 7). However, such a composition for coating materials neither has considerably high gas-barrier property nor exhibits high gas-barrier property under high-humidity conditions. Therefore, demand has arisen for further improvement in gas-barrier property.
Also, there has been proposed an epoxy resin composition exhibiting high gas-barrier property, which contains an epoxy resin having a specific structure, and an amine-containing curing agent (see Patent Document 8). However, the aforementioned epoxy resin composition is unsatisfactory in terms of short pot life and poor workability, although the composition exhibits good gas-barrier property, adhesion, and chemical resistance.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. H09-77089
Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. H09-314735
Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. H10-34821
Patent Document 4: Japanese Patent Application Laid-Open (kokai) No. 2001-98047
Patent Document 5: Japanese Patent Application Laid-Open (kokai) No. 2004-231730
Patent Document 6: Japanese Patent Publication (kokoku) No. H07-91367
Patent Document 7: Japanese Patent Publication (kokoku) No. H07-91368
Patent Document 8: Japanese Patent Application Laid-Open (kokai) No. 2002-256208